Pathogenic eukaryotes in the phylum Apicomplexa include some of the most notorious protozoan parasites, such as Plasmodium (malaria), Cryptosporidium (diarrheal disease), and Toxoplasma (toxoplasmic encephalitis). The latter two parasites have recently been added to the NIAID watch list of Category B pathogens of interest for Biodefense, but have long been serious opportunistic infections in AIDS and other immunosuppressed patients. The standard treatment for toxoplasmosis is limiting due to toxic adverse effects, underscoring the need for new drug target discovery. Epigenetic modifications such as histone acetylation have previously been validated as a novel drug target in apicomplexan parasites, but we understand very little about these processes. We have been working to remedy this deficiency through the characterization of the histone acetyltransferases (HATs) in the Toxoplasma parasite. HATs in the MYST family function in multi-subunit complexes, preferentially acetylate histone H4, and are generally essential for viability. We have recently cloned and characterized two MYST HATs in Toxoplasma (TgMYST-A and -B) and generated specific antibody to each. We have discovered that the level of MYST HAT activity in the parasite is critical for parasite growth. Additionally, many of the well conserved components of MYST HAT complexes in other species are not present in the Toxoplasma genome. In short, our preliminary data suggests that the two MYST HATs we have identified in Toxoplasma may be essential and are likely to operate in HAT complexes composed of unique parasite-specific proteins. Therefore, we hypothesize that TgMYST-A and -B form HAT complexes that are essential for Toxoplasma viability. The main objective of this R21 is to address this hypothesis by 1) Evaluating the impact of TgMYST HATs on parasite physiology by generating conditional knockouts for TgMYST-A and -B and assessing the consequences on parasite viability, and 2) Identifying components of each TgMYST HAT complex using biochemical strategies to purify proteins associating with TgMYST-A and -B. The reagents and data generated from this R21 promise to facilitate the discovery of new drug targets for treatment of toxoplasmosis and potentially other apicomplexan infections. They will also have great value as tools to fill the gaps in our knowledge of HAT-mediated gene regulation in parasites with the long term goal of exploiting this vital process pharmacologically. PUBLIC HEALTH RELEVANCE: It is in the interest of public health to investigate novel drugs and drug targets against Toxoplasma gondii, a parasite that causes congenital birth defects and opportunistic infection in AIDS and other immunosuppressed patients. Toxoplasma is also listed by NIAID as a category B pathogen relevant to Biodefense research. We propose to examine enzyme complexes that are involved in parasite gene regulation and determine if they are promising targets for drug development.